1. Field of the Invention
The present invention relates to atomic and molecular beams and more particularly to an apparatus and method for producing controllable low energy neutral atomic and molecular beams.
2. Description of the Prior Art
With the increasing use of low earth orbit vehicles and satellites, problems with the residual atmosphere in such orbits has surfaced as evidenced by significant crosion of exposed surfaces. The environment in the region of low earth orbits at 300-500 km is primarily atomic oxygen. Although there are few atoms at this altitude, the high velocity of the orbiting payloads and the high reactivity of atomic oxygen degrade many organic surface materials at a high rate. The erosion is most severe on organic polymers containing carbon, hydrogen, oxygen, nitrogen and sulfur. See the following reports: L. J. Leger, J. T. Visentine, and J. A. Schliesing, "A consideration of atomic oxygen interactions with space station", AIAA-85-0476, Proc., AIAA 23rd Aerospace Sciences Meeting, Reno, NV, Jan. 14-17, 1985 and A. F. Whitaker, S. A. Little, R. J. Harwell, D. B. Griner, R. F. Dehaye, and A. T. Fromhold, Jr., "Orbital atomic oxygen effects on thermal control and optical materials--STS-8 results", AIAA-85-0416, Proc. AIAA 23rd Aerospace Sciences Meeting, Reno, NV, Jan. 14-17, 1985.
It is necessary to be able to predict the useful lifetime of components in the design of space stations and other space vehicles. In the past, various materials have been investigated by exposing materials in low earth orbit and recovering the materials for analysis. This approach is costly in time and money and opportunities for such testing are limited. It is therefore highly desirable to be able to simulate the low earth orbit environment in the laboratory.
The predominant constituent of this environment, as mentioned above, is atomic oxygen formed by the photodissociation of molecular oxygen. The atomic oxygen, at densities of 10.sup.7 -10.sup.9 atom/cm.sup.3, has only thermal energy. However, a spacecraft traveling at a velocity of 8 km/s experiences a flux of 10.sup.13 -10.sup.15 atom/cm.sup.2 s with an average atomic energy of 5 ev. The material surface also can be exposed to ultraviolet (UV) radiation depending on its orientation, and a flux of nitrogen gas molecules depending on its altitude. Ideally, laboratory tests would allow atomic oxygen, nitrogen molecules, and UV photons to be incident on samples both separately and in the various combinations. However, the major problem is the generation in the laboratory of the flux of atomic oxygen which is believed to be the most damaging of these several factors.
Such a flux of atomic oxygen has not been readily available in the laboratory in the prior art for a variety of reasons: atomic oxygen is not stable against recombination; the fluxes desired are relatively high; neutral beams are difficult to manipulate; and 5 eV ions are too low in energy to easily focus and velocity select, but too high in energy to generate thermally. See J. B. Cross and D. A. Cremers, "Atomic oxygen surface interactions-mechanistic study using ground-based facilities", AIAA-85-0473, Proc. AIAA 23rd Aerospace Sciences Meeting, Reno, NV, Jan. 14-27, 1985.
A need exists for methods to produce a controlled low energy beam of atomic oxygen in the laboratory to permit simulation of low earth orbit environments.